1. Field of the Invention
This invention relates to an improved method for the purification of N-acetyl-para-aminophenol (APAP), also known as acetaminophen. APAP is a well-known over-the-counter analgesic and anti-pyretic agent.
2. Description of Related Art
The following prior art references are disclosed in accordance with the terms of 37 CFR 1.56, 1.97 and 1.98.
U.S. Pat. No. 3,042,719, issued Jul. 3, 1962 to Hahn et al., discloses the purification of crude discolored APAP by acidifying an aqueous solution of the APAP with a mineral acid, filtering the solution while hot, and cooling the filtrate while adding an alkaline reducing sulfite, e.g., sodium hydrosulfite (sodium dithionite). A "decolorizing" carbon may be added to the hot solution.
U.S. Pat. No. 3,113,150, issued Dec. 3, 1963 to Young, teaches the preparation of "pure" APAP by adding acetic anhydride to a mixture of p-aminophenol and water, cooling the reaction mixture to precipitate the APAP, filtering to remove excess acetic acid, neutralizing the wet APAP with ammonium hydroxide, and agitating the resulting solution with carbon black.
U.S. Pat. No. 3,748,358, issued Jul. 24, 1973 to Baron, discloses the purification of APAP by treating it in aqueous solution with carbon which has been preliminarily treated with an acidic solution.
U.S. Pat. No. 3,781,354, issued Dec. 25, 1973 to Kosak, teaches the purification of APAP by treating it in hot aqueous solution with ferric chloride and adsorbing the colored by-product on activated carbon.
U.S. Pat. No. 4,524,217, issued Jun. 18, 1985 to Davenport et al., teaches an integrated process for the production of APAP comprising acetylating phenol by a Friedel-Crafts reaction, or subjecting phenyl acetate to a Fries rearrangement to produce 4-hydroxyacetophenone (4-HAP), reacting the 4-HAP with hydroxylamine or a hydroxylamine salt to form 4-HAP oxime, and subjecting the latter oxime to a Beckmann rearrangement to form APAP.